AKAramis said:1st, because it's possible (maybe even likely) that populations up to the millions could (and will) live on totally non-habitable worlds. Why? Because once you get past a couple generations you develop a local identity, and that is part of habitation patterns. So long as the basic needs (Food, water, atmosphere, stimulation) are met, people will live there.
And I've already said that I strongly disagree with that opinion, because the studies you referenced aren't remotely the same situation.
People lived in various catacombs.
When was the last time millions of people lived in a single cavern system?
But plain and simple, Traveller is Space Opera... and as a genre, that includes the hive worlds, dome habs, and other types of high density high population enclave that are "unlikely" by some peoples view points.
Except it's not Space Opera. Not pure-blooded space opera anyway - it's got a strong bent of grittiness and realism in it. It's more Alien than Star Wars.
Vulcan in Sten, a size 1 world long since buried under the industrial complex that grew up over it. Population somewhere around a million.
I don't know anything about this, but I will say that Star Trek is not even remotely the similar to Traveller.
[*]Venus and Mars colonies from Jovian Chronicles - both present due to terraforming efforts, and both in the 10E7 range.[*]Mars colonies in Babylon 5[*]Mars in Total Recall
All of which are worlds within the same system as a habitable homeworld, so aren't really valid examples. And again, those aren't Traveller and don't hold Traveller's tropes.
They may not be realistic, but the system has to allow for them, as they are expected.
Expected by whom? The system doesn't HAVE to allow for them at all. How about Hadley's Hope on Acheron? What about Io from Outland? Or the prison colony from Alien 3? Those are arguably a lot closer to the feel of Traveller than anything you quoted.
As for B & C, no, I disagree. It is entirely possible that no other worlds exist, or that there is some reason the world is being inhabited.
Then build a space station - it's a lot better and cheaper than living on the surface. Especially when there's other worlds nearby (even an asteroid in the same system is easier to live on than that kind of environment). For one thing you don't need to keep repairing vacc suits and vehicles all the time just because they're outside for a few hours...
That's another thing that everyone forgets about, incidentally - the actual cost of living in such a hostile environment. If you had a choice of funding a colony (because someone has to pay for it, and for getting there, even though it's totally ignored in Traveller) then would you rather choose to build wooden huts or unsealed buildings on a habitable world, or high tech hermetically sealed habitats that need a ton of automation and computers to run properly? It's vastly cheaper to take the habitable option, and also those worlds are much more attractive to people so you can get more colonists wanting to get out there. Economically speaking (never mind anything else), it strikes me as being utterly insane to live in an environment as harsh as a B or C atm.